1. Field of the Invention
This invention relates to the preparation of new strains of a toxin-producing bacterium having entomocidal activity, a new strain thereby produced and the use of this bacterium to protect plants susceptible to certain pests.
2. Discussion of the Background
More particularly, this invention relates to a new strain of Bacillus thuringiensis which has an improved entomocidal activity against certain lepidopterous pests.
Many plants of commercial and/or domestic importance are subject to invasion and damage by lepidopterous pests. These pests are found throughout the world and currently their control requires repeated and costly application of the pesticides which are available.
Many crops are susceptible to a number of lepidopterous pests. Among the most significant pests are the cotton leafworm (Spodoptera littoralis), the bollworm (Heliothis armigera), the tobacco budworm (Heliothis virescens), Plutella maculipennis, Mamestra brassicae and Pieris brassicae.
Broad spectrum pesticides are a useful and valuable tool for crop protection, but the indiscriminate use of broad spectrum chemical insecticides can disrupt many natural control agents. Since most chemical insecticides are relatively non-selective, they may destroy non-target organisms, including beneficial predators and parasites of destructive pests. Some insects also develop resistance to chemical insecticides, which often make them uncontrollable by these insecticides and may permit minor pests to become major ones.
The introduction of selective microbial insecticides using naturally occurring bacteria as the active and host-specific ingredient has helped to overcome many of these problems.
An example of a microbial insecticide is Bacillus thuringiensis, a number of strains of which are commercially available and are currently exploited for their unique insecticidal activity when eaten by susceptible larvae, particularly those of the insect order Lepidoptera.
These strains may be employed without ill effect on beneficial insects. Bacillus thuringiensis fits well into current agricultural theories which support the use of naturally occurring organisms to suppress harmful insects. It is a widely distributed, rod-shaped, spore forming, aerobic, gram positive micro-organism and is characterised by producing, during the sporulation cycle, one or more proteinaceous parasporal crystals; its pathogenicity for lepidopterous larvae; its ability to use citrate as its sole source of carbon; and the exceptionally high phosphate content of its spore.
Bacillus thuringiensis is a common inhabitant of the environment and is capable of growth in certain types of soils. It has no known adverse effect on life forms such as man, pets, birds, fish, earthworms, most beneficial insects or plants. Its pathogenicity to sensitive insects is essentially due to the presence of a parasporal crystal, which may represent 30 to 40% of the dry weight of the cell at the time of sporulation.
Bacillus thuringiensis is active only when ingested. Some hours after ingestion has occurred lepidopterous pests cease to feed and damage to the plant is stopped. Most species die after approximately 24 to 72 hours from toxaemia due to the crystal toxins. This is sometimes accompanied by septicaemia as a result of the presence of the spore.
Thus, the principal effect is due to the crystal which acts only after its dissolution in the intestines of the larvae.
The activation of the crystal is caused by a combination of alkaline pH and proteolytic enzymes in the gut contents. The reaction is dependent on the high gut pH of lepidopterous larvae (pH &gt;7), which allows the release of the toxic components of the crystal. These toxins break down the mid-gut wall causing feeding to stop.
The growth of bacteria thus released into the abdominal cavity results in septicaemia which also may play a part in the death of the insect.
It is clear that the use of Bacillus thuringiensis as an insecticide provides an effective and environmentally acceptable method of dealing with lepidopterous pests.
For this reason new strains having improved insecticidal activity, either in terms of greater toxicity for given species or in terms of a broader spectrum of activity, are currently being sought. However, the combination of high toxicity and broad spectrum of activity has in practice been very difficult to achieve.